1. Field of the Invention
The invention relates generally to a vehicle differential unit, and more specifically to a vehicle differential unit that includes a support member that rotatably supports a gear member.
2. Description of the Related Art
For example, Japanese Patent Application Publication No. 2005-16643 describes an existing vehicle differential unit that includes: a housing having a planetary carrier that rotatably houses and supports planetary gears and that serves as an input member; an internal gear that meshes with the planetary gears in the planetary carrier and that serves as a first output member; and a sun gear that is coaxial with the internal gear, that meshes with the planetary gears and that serves as a second output member.
The housing receives a torque from an engine of a vehicle and rotates together with the planetary carrier, and the rotational force is transmitted from the planetary carrier to the sun gear and the internal gear via the planetary gears.
The internal gear has helical splines that mesh with a coupling. The internal gear receives the rotational force of the planetary gears and rotates, and then transmits the rotational force to the coupling.
The coupling has helical splines that form a thrust force generation portion together with the helical splines of the internal gear, and transmits the rotational force from the internal gear to a rear axle (output shaft).
The sun gear receives the rotational force of the planetary gears and rotates, and then transmits the rotational force to a front axle (output shaft).
With the structure described above, when the torque from the engine of the vehicle is input in the housing, the housing rotates about its rotational axis. When the housing rotates, the rotational force is transmitted to the planetary gears via the planetary carrier, and then transmitted from the planetary gears to the internal gear and the sun gear.
In this case, when the vehicle travels straight, a front drive wheel and a rear drive wheel rotate at the same speed. Therefore, the planetary gears mesh with the sun gear and the internal gear and do not rotate relative to the sun gear and the internal gear. As a result, the entirety of the differential unit revolves around the rotational axis of the housing.
On the other hand, when a difference in the rotational speed between front drive wheel and the rear drive wheel is caused, for example, when the vehicle goes round a curve, the planetary gears rotate about their axes within the planetary carrier while revolving around the rotational axis of the housing so as to adjust the difference in the rotational speed. That is, the planetary gears rotate on their axes so that the rotational speed of one of the sun gear and the internal gear is increased and the rotational speed of the other of the sun gear and the internal gear is reduced, whereby the differential function is performed.
When a difference in the grip force between the front drive wheel and the rear drive wheel is caused, the differential limiting operation is performed using the friction force generated due to the thrust force that is generated by the mesh of the planetary gears with the sun gear and the internal gear or using the friction force generated between the tooth tip faces of the planetary gears and the gear support face of the planetary carrier. At this time, the differential limiting force is amplified or reduced by the thrust force that is generated at the thrust force generation portion between the internal gear and the coupling.
However, with the vehicle differential unit according to JP-A-2005-16643, when the vehicle is in the drive mode or the coast mode, a thrust force is generated at the thrust force generation portion, and the differential limiting force is increased or reduced. Therefore, it is not possible to make an adjustment of the differential limiting function (bias ratio) that is limited to one of the drive mode and the coast mode.
For example, if the locking factor performance in the coast mode is as high as that in the drive mode, the differential limiting force exerts an influence on the operation of a so-called anti-lock brake system that prevents locking of a wheel when the vehicle is decelerated by a brake, for example, on a road having a low frictional coefficient μ. As a result, the performance of the anti-lock brake system may not be sufficiently delivered. Also, in a vehicle provided with a vehicle behavior stabilization control device that corrects an unstable behavior of the vehicle that may occur while the vehicle is making a turn or a device that prevents spinning of a wheel during sudden acceleration, if the locking factor in the drive mode is increased, the differential limiting force interferes with the operations of these devices. As a result, these devices may not sufficiently deliver their performance. Even if a vehicle is not provided with these devices, it is preferable that the locking factor differs between the coast mode that is mainly selected when the vehicle travels straight and the drive mode that is frequently selected when the vehicle goes through a curve. That is, the locking factor in the drive mode and the locking factor in the coast mode have respective optimum values based on the structure of the vehicle. In order to achieve the optimum values, the locking factor in the drive mode and the locking factor in the coast mode need to be made different from each other. However, in the above-described existing vehicle differential unit, such a measure is not taken.